In-situ oxygen detector

ABSTRACT

A measuring apparatus having a self-contained reference atmosphere and utilizing thick film thermocouples to measure the operating temperature of the detector cell. An embodiment of the detector is provided with heating means and enclosed so as to be capable of operation in a high velocity varying temperature atmosphere. In another embodiment, the operating temperature of the detector cell is measured by one thick-film thermocouple inside and another outside the closed end of the electrolyte tube of the detector. The outputs of the two thermocouples are electrically parallel connected to provide an average of the two thermocouple readings.

Unite States tet 1 Flais et a1.

[451 Mar. 18, 1975 iN-siTu OXYGEN DETECTOR [75] Inventors: Louis R.Flais; Ralph G. Gentle,

both of Alliance, Ohio [73] Assignee: Bailey Meter Company, Wickliffe,

Ohio

[22] Filed: July 2, 1973 [21] Appl. No.: 376,053

Related US. Application Data [62] Division of Ser. No. l76,804, Sept. l,1971, Pat. No.

[52] US. Cl. 204/195 S [51] int. Cl. G01n 27/46 [581 Field of Search204/195 S, 1 T; 324/29 [561 References Cited UNITED STATES PATENTS3,576,730 4/1971 Spacil 204/195 S 3,597,345 8/1971 Hickham et a]. 3598,7ll 8/1971 Flais 204/195 S FOREIGN PATENTS OR APPLICATIONS 37,8005/1965 Germany 204/195 S Primary Examiner-G. L. Kaplan Attorney, Agent,or FirmJ. M. Maguire [57] ABSTRACT A measuring apparatus having aself-contained reference atmosphere and utilizing thick filmthermocouples to measure the operating temperature of the detector cell.An embodiment of the detector is provided with heating means andenclosed so as to be capable of operation in a high velocity varyingtemperature atmosphere. In another embodiment, the operating temperatureof the detector cell is measured by one thickfilm thermocouple insideand another outside the closed end of the electrolyte tube of thedetector. The outputs of the two thermocouples are electrically parallelconnected to provide an average of the two thermocouple readings.

6 Claims, 6 Drawing Figures HEATER CONTROL SHEET 2 BF 3 HEATER CONTROLIN-SITU OXYGEN DETECTOR This is a division, of application Ser. No.176,804, filed 9/1/71 now US. Pat. No. 3,767,469.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to improvements in apparatus for in-situ detection andcontinuous measurement of the oxygen content in gases and moreparticularly to a device for flue gas monitoring having a referenceatmosphere which limits oxygen migration if any to the direction of thereference only.

2. Description of the Prior Art As described in US. Pat. No. 3,598,711titled Electrochemical Oxygen Analyzer, an oxygen detection cell can becalibrated directly from the Nernst equation which relates the voltageoutput of the detector cell to a function which is a product of celltemperature and the log of the ratio of oxygen concentration across thecell. By providing an atmosphere of known oxygen concentration on oneside of the cell and measuring cell temperature and cell voltage output,the oxygen concentration on the other side of the cell is easilycalculated, and the need for calibration with bottled gases of knownconcentrations is eliminated. Atmosphere or bottled gas of knownconcentration, however, is still required during operation for use as aconstant oxygen partial pressure reference on one side of the cell. Theimprovement herein eliminates the need for bottled reference gas byproviding a self-contained, sealed metal-metal oxide reference system.

One of the principal problems with a sealed reference system including agaseous mixture is that it is normally I susceptible to migration ofoxygen through the electrolyte tube causing a change in reference orsample gas oxygen partial pressure. Such a problem is present in thedevice taught by Kolodney et al. in U.S. Pat. No. 3,481,855 for a liquidmetal oxygen monitor. Another problem with cells of the type taught inBritish Pat. No. 1,081,545, published May 19, 1965, is that sealing ofthe reference system is done with glass compounds which soften and flowat temperatures much lower than the 1500F operating temperature of somedetectors. Furthermore, the SiO in a glass seal reacts with the oxygenin the reference system thus changing the reference oxygen partialpressure to the detriment of cell accuracy and reliability.

Calibration without the need for atmosphere or bottled gases of knownoxygen concentration is extremely dependent upon the accuratemeasurement of cell operating temperature to yield an accurate solutionfor unknown oxygen concentration. This measurement is complicated by thepresence ofa temperature gradient which exists across the thickness ofthe electrolyte tube. One temperature is defined on the outsideelectrode, and a different temperature is defined on the insideelectrode. The problem is further aggravated by the difficulty ofobtaining good contact between the temperature sensing means and theelectrode temperature being measured. Good contact between the sensorand the area sensed is needed since otherwise the most accuratetemperature measuring means will give an inaccurate reading.

SUMMARY OF THE INVENTION In the accordance with the present inventionthere is provided a zirconium-oxide tube with a compartively lowreference oxygen partial pressure relative to that of the sampled gas,and a metal-metal oxide reference system is sealed inside the tube by abrazed zirconia cap located over its open end. A pair of platinumelectrode strips, one inside and the other outside of the tube, overlapin the central area of the closed end of the tube, to provide limitedarea electrodes of constant temperature to sense the voltage developedacross the electrolyte.

Further in accordance with the invention, a thickfilm thermocouple inmolecular contact with the electrolyte tube senses the outside electrodetemperature.

Further in accordance with the invention, temperature gradients acrossthe thickness of the zirconia tube are eliminated to provide an 'in-situdetector for high velocity atmospheres of flucatuating temperature byenclosing the electrolyte tube so as to leave only the closed end of thetube accessible to the unknown oxygen atmosphere. A first heating meansin the enclosure heats a part of the lengh of the electrolyte tube nearthe closed end, and a second heating means in the enclosure opposite theclosed tube end heats the unknown atmospheres as well as the closed endof the tube.

Further in accordance with the invention, electrode operatingtemperatures are sensed by two thick-film thermocouples, one sensing theexternal electrode temperature and the other sensing the internalelectrode temperature, in situations where there is a significanttemperature gradient across the thickness of the zirconia tube. Thethermocouples are electrically connected to provide an average of thetwo temperatures sensed thus giving a good indication of the averagecell operating temperature. The platinum electrode may serve as onethermoelement of the thick-film thermocouple thus functioning in a dualcapacity. The other thermoelement of the thick-film thermocouple may beone of a number of other elements consisting of gold, iridium,palladium, and MoSi An alloy of Pt and Rh may also be used such as Pt10% Rh or 87% Pt-l3% Rh.

This invention eliminates the effects of oxygen migration by providing asealed reference inside of the electrolyte tube having a lower oxygenpartial pressure than that of the oxygen partial pressure outside thetube so that oxygen migration is only possible toward the referenceside. This invention also teaches that a free metal is provided to reactwith the migrating oxygen and form the metal oxide thereby maintaining aconstant predetermined oxygen content in the reference.

The invention also provides a high temperature seal able to withstand1500F operating temperatures. This is accomplished by brazing a zirconiacap on the open end of the zirconia electrolyte tube after the referencesystem is inserted therethrough.

The invention herein solves the temperature measurement problem byproviding an average reading of cell wall temperatures where atemperature gradient is prevalent thus obviating the need to locate athermocouple inside of the cell wall. This is accomplished by sensinginternal and external wall temperatures at the electrodes withthermocouples and electrically connecting the thermocouple outputs inparallel to give an average reading of the cell operating temperature.

The invention also provides a means of eliminating the gradient acrossthe wall of the electrolyte cell to cancel the associated thermoelectricvoltage effects by a unique enclosure andheating system which providesone set of heaters for the body of the electrolyte tube and another setfor the closed end of tube and the atmosphere in contact with it.

The invention also solves the problem of accurate cell temperaturemeasurement by the use of a thickfilm thermocouple which is in molecularcontact with the sensed surface.

The simultaneous use of the platinum electrode of the detector as anelectrode and also as one thermoelement of the thick-film thermocoupleis also an important feature.

The principal object of the invention, therefore, is to provide anin-situ oxygen detector which does not require either access toatmosphere or bottled gas for callibration or for the reference systemand which gives sustained accurate readings of oxygen content in anunknown gas because of improved temperature sensing methods andelimination of temperature gradients.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a top view representation ofan in-situ oxygen detector utilizing a thick-film thermocouple andsealed reference atmosphere.

FIG. 2 is a cross-sectional elevational view taken along section 2-2 ofthe in-situ oxygen detector of FIG. 1 rotated 90 out of the plane of thepaper.

FIG. 3 is a schematic of a system for controlling the temperature of anelectrolyte cell using internal and external thick-film thermocouples.

FIG. 3(a) is a chart representation of a gold, platinum thick-filmthermocouple voltage output as a function of the thermocouples junctiontemperature.

FIG. 4 is a top view representation of a jacketed insitu oxygen detectorfor atmospheres of high velocity and varying temperature taken alongsection 4-4 of FIG. 5.

FIG. 5 is a cross-sectional view taken along section 5-5 of the jacketedoxygen detector of FIG. 4 rotated 90 out of the plane of the paper.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGS. 1 and 2, aself-contained reference sensor cell comprises a zirconium-oxideelectrolyte tube 16 having a closed end 17, the central portion of whichlocates a region of overlapping electrodes 34. The overlappingelectrodes along with the intervening zirconium-oxide tube forms anoxygen concentration cell. The overlapping electrodes include a platinumelectrode strip 22 on the outside of the tube and a platinum electrodestrip 30 inside the tube. The outside strip 22 extends partially acrossthe outside of the electrolyte closed end 17 and continues along theoutside length of the electrolyte tube 16 terminating in electricalcontact at an electrode contact band 24. The inside strip 30 extendspartially across the inside of the electrolyte closed end 17 ancontinues along the full inside length, over the open end and partiallyover the outside length of the electrolyte tube 16 terminating inelectrical contact at an electrode contact band 32.

On the outside of the electrolyte closed end 17 there is formed athick-film thermocouple having a gold thermoelement 26, and the platinumelectrode strip 22 is used as the other thermoelement of dissimilarmetal. For the purposes of this disclosure, a thick-film thermocouple isappproximately from 0.] to 1.0 mils thick and is applied to the surfacewhose temperature is to be measured as a paste which is then fired in anoven to obtain metallic charactertistics. A paste typical of the onedescribed is manufactured and may be obtained from Englehard Industries.The gold thermoelement 26 extends perpendicular and partially across theelectrolyte closed end 17 to join the end of the outside electrode strip22 and form a thick-film thermocouple junction 29 there. The goldthermoelement then continues along the outside length of the electrolytetube 16 to electrically contact a gold thermoelement contact band 28. Arefractory film such as SiO may be used to electrically isolate the goldthermoelement from electrical contact with the contact band 24.

The inside of the electrolyte tube 16 is filled with a metal-metaloxide, reference system 18, such as copper-copper oxide, through thetube open end 19 which is thereafter sealed by brazing a zirconium-oxidecap 12 to the electrolyte tube 16 with a high temperature braze alloy 14such as cupric oxide-silver, silver-copper-palladium, gold-palladium,nickel-titanium, or other commercial nickel alloys.

In operation, the sensor 10 is directly inserted into a high constanttemperature atmosphere of unknown oxygen content. The sensor operates asan oxygen concentration cell whose open circuit voltage output is givenby the Nernst equation below:

Where:

E open-circuit potential (volts) F Faraday number (96,500 coulombs) 4Fquantity of charge transferred per mol of 0 passing through theelectrolyte (386,000 coulombs/mol) R universal gas constant (8.314watt-sec/K mol) T absolute temperature of cell (K) P 0 P 0 partialpressure of oxygen in the two cell chambers In the detector described,the overlapping electrodes and intervening zirconium-oxide form theoxygen concentration cell. Thus when there is a different oxygenconcentration on the outside of the electrolyte closed end 17 from thaton the inside of the closed end 17, a voltage potential E will be seenon a voltage indicator 38 connected across the electrode band 24 and theelectrode band 32. v

A temperature sensing means, regardless how accurate, will give aninaccurate reading if there is relatively poor contact between itselfand the surface being measured. The gold-platinum thick-filmthermocouple as described herein provides molecular contact between theelectrolyte closed end 17 and the gold and platinum thermoelements 26,22 respectively. Consequently the temperature sensed by the thermocouplejunction 29 is an extremely accurate temperature measurement,representative of the cell operating temperature T in the Nernstequation. A temperature indicator 36 connected across the electrodecontact band 24, here serving also as the contact band for the platinumthermoelement of the thick-film thermocouple, and a contact band 28,connected to the gold thermoelement 26, provides a means of continuallyindicating this coil temperature T.

The sealed metal-metal oxide reference system 18 provides a constantoxygen partial pressure for a specific cell temperature T as determinedby the metalmetal oxide equilibrium. More specifically, a coppercuprousoxide reference system will provide an oxygen partial pressure ofatmospheres at 800C. This partial pressure will be maintained even ifthere is a migration of oxygen through the electrolyte tube into thereference system 18 since the migrating oxygen will react with the metalof the reference system to form the oxide and maintain the oxygenpartial pressure constant. Since the oxygen partial pressure of thereference system 18 is lower than that outside of the electrolyte tube16, notwithstanding that the differential pressure may be contra theretoin certain applications, oxygen migration is exclusively unidirectionaltoward the reference system 18.

The cell operating temperature T is accurately measured and the oxygenpartial pressure on the one side of the cell kept constant; the voltageof indicator 38 becomes a direct reading instrument for the oxygenpartial pressure of the unknown atmosphere outside of the electrolytetube 16. Any requirement for bottled gas of known oxygen concentrationis obviated since it is necessary neither for calibration nor to keep aconstant oxygen partial pressure on one side of the cell.

Refer now to the system of FIG. 3 and assume the electrolyte tube 16 issubjected to a significant temperature gradient across the closed end17. The temperature sensed by a thermocouple .on the outside of theelectrolyte tube 16 is a high reading and a thermocouple on the insideis a low reading. Neither reading taken alone is a good representationof the oxygen concentration cell operating temperature. An accuratetemperature reading of the cell is needed. A thick film thermocouple 41having a platinum thermoelement 40 and a gold thermoelement 39 is bondedto the outside of the electrolyte tube 16 and a thick-film thermocouple43 having a platinum thermoelement 44 and a gold thermoelement 42 isbonded to the inside of the electrolyte tube 16 opposite thethermocouple 41. The outputs of the inside and outside thermocouples areparallel-connected to a temperature indicator 36 through thermocoupleoutput lines 58, 56. Through this electrical connection the reading ofthe indicator 36 isthe average of the temperatures sensed by thethermocouples 41, 43. This indication of the average cell operatingtemperature is transmitted along input lines 54 to a heater control 48which operates a supplementary heater 46 for the electrolyte tubethrough heater power lines 50. The heater control 48 is supplied bypower supply lines 52. This heater is used in atmospheres of low orfluctuating temperature.

In operation the heater control has a set point temperature againstwhich the average temperature signal of the electrolyte cell iscompared. If this temperature reading of the cell is below the setpoint, the heater control activates the heater 46 until the temperaturesare equalized.

Referring now to FIG. 30, it should be noted that the voltage output vstemperature curve for the goldplatinum thick-film thermocouple ispractically linear beyond 600F. Since the oxygen concentration cell isoperated at temperatures in excess of 600F, this type of thermocouplebecomes particularly applicable and accurate.

Referring now to FIGS. 4 and 5, ajacketed sensor assembly 67 is shownhaving unknown atmosphere passageways 62 in a detector jacket 60.Parallel flow throttling baffles 72 are shown inside the assembly 67 toenclose the sensor cell 10. The sensor cell 10 is retained inside thejacket 60 so as to have its closed end 17 exposed to the unknownatmosphere chamber 71 and the length of its body sealed inside thejacket 60 by a labyrinth of seals 74. The closed end 17 portion of theelectrolyte tube is maintained at a constant temperature by acircumferential electrolyte heater 66 which is located circumferentiallyoutside the jacket inside wall 77. This heater 66 is insulated from thedetector jacket 60 by Kaowool heater insulation 64 within the interiorspace therein. The atmosphere chamber 71 is maintained at operatingtemperature by an end heater 68 which is also insulated from the jacket60 by Kaowool insulation 70. The circumferential heater 66 and the endheater 68 are controlled by a heater control 76 through control lines78, respectively. Power to the heater control 76 is supplied throughcontrol power lines 82.

In operation, the jacketed assembly 67 is extended into an atmosphere ofunkown oxygen concentration. The atmosphere is able to enter and exitthe atmosphere chamber 71 through passageways 62. Once inside thechamber 71 the entering air stream is throttled by the baffles 72 whicheven out air flow and enhance air stream contact with the closed end 17of the electrolyte tube 16 forming the oxygen concentration cell. Theair stream temperature is maintained constant by the end heater 68providing supplemental heat when necessary to bring the air stream up tooperating temperature. Meanwhile, the circumferential heater 66 ismaintaining the electrolyte closed end 17 at a constant operatingtemperature by supplying heat to the electrolyte tube to maintain theoperating temperature. Because the electrolyte tube is insulated anduniformly heated and the air stream entering the chamber 71 is beingpreheated and throttled, the temperature gradient across the closed end17 and the accompanying thermoelectric effects are substantiallyeliminated. Thus this type of insitu detector is extremely well adaptedto sense oxygen content in high velocity, atmospheres of varyingtemperature.

Various modifications will become obvious to persons skilled in the artupon reading this specification. As an example of an obviousmodification, the system of inside and outside thermocouples could bemodified to provide a signal upon attaining a certain minimaldifferential temperature thus showing that the 0 cell has stabilized andis ready for operation. Also the temperature sensing signal may be fedthrough an analog multiplier and into a summing circuit with thedetector cell output to directly compensate for the temperaturevariations of the detector cell. It is our intention that such variousmodifications also be incorporated.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. An in-situ oxygen detector comprising an electrolyte tube ofzirconium oxide formed so as to have one end open and one end closed;

a first porous platinum electrode strip bonded to the outside of saidtube, extending partially across the closed end and continuing along theside of said tube to a first contact terminal on the outside of saidtube;

a second porous platinum electrode strip bonded to the inside of saidtube, extending partially across the closed end of said tube so as tooverlap said first electrode and continuing along the inside, over theopen end and partially on the outside of said tube to a second contactterminal on the outside of said tube;

means sealing the open end of said tube including a zirconia cap;

a self-contained reference atmosphere sealed inside said tube by saidsealing means;

a first thermocouple located on the outside closed end of saidelectrolyte tube so as to detect the temperature of said firstelectrode; and

a second thermocouple located on the inside closed end of saidelectrolyte tube so as to detect the temperature of said secondelectrode.

2. An in-situ oxygen detector as set forth in claim 1 wherein said firstand second electrode strips overlap in the central portion of the closedend of said tube so as to form an electrode pair for ion conduction onthe center of the closed end of said tube.

3. An in-situ oxygen detector as set forth in claim 2 wherein saidself-contained reference atmosphere includes a metal-metal oxide powder,said metal reacting with any oxygen migrating through saidzirconiumoxide tube to form the metal oxide and thereby maintaining saidreference atmospheres oxygen partial pressure constant.

4. An in-situ oxygen detector as set forth in claim 3 wherein saidsealing means includes the brazing of said zirconia cap to saidelectrolyte tube with a braze alloy capable of maintaining said brazedseal at 1500F ambient temperature.

5. An in-situ oxygen detector as set forth in claim 1 wherein said firstand second thermocouples are thickfilm thermocouples from 0.1 to 1.0mils thick 6. An in-situ oxygen detector as set forth in claim 1including:

means for enclosing said electrolyte tube so as to leave the closed endof said tube exposed to an atmosphere of unknown oxygen content;

a first means for heating a portion of the enclosed length of saidelectrolyte tube near the closed end of said tube, said first heatingmeans being located along the length of said tube and internal to saidenclosing means;

a second means for heating the unknown atmosphere and the closed end ofsaid tube located opposite the closed end of said tube; and

means for controlling said first and second heating means so that thetemperature sensed by said first and second thermocouples is maintainedat a substantially indentical temperature level.

1. AN IN-SITU OXYGEN DETECTOR COMPRISING AN ELECTROLYTE TUBE OFZIRCONIUM - OXIDE FORMED SO AS TO HAVE ONE END OPEN AND ONE END CLOSED;A FIRST POROUS PLATINUM ELECTRODE STRIP BONDED TO THE OUTSIDE OF SAIDTUBE, EXTENDING PARTIALLY ACROSS THE CLOSED END AND CONTINUING ALONG THESIDE OF SAID TUBE TO A FIRST CONTACT TERMINAL ON THE OUTSIDE OF SAIDTUBE; A SECOND POROUS PLATINUM ELECTRODE STRIP BONDED TO THE INSIDE OFSAID TUBE, EXTENDING PARTIALLY ACROSS THE CLOSED END OF SAID TUBE SO ASTO OVERLAP SAID FIRST ELECTRODE AND CONTINUING ALONG THE INSIDE, OVERTHE OPEN END AND PARTIALLY ON THE OUTSIDE OF SAID TUBE TO A SECONDCONTACT TERMINAL ON THE OUTSIDE OF SAID TUBE; MEANS SEALING THE OPEN ENDOF SAID TUBE INCLUDING A ZIRCONIA CAP; A SELF-CONTAINED REFERENCEATMOSPHERE SEALED INSIDE SAID TUBE BY SAID SEALING MEANS; A FIRSTTHERMOCOUPLE LOCATED ON THE OUTSIDE CLOSED END OF SAID ELECTROLYTE TUBESO AS TO DETECT THE TEMPERATURE OF SAID FIRST ELECTRODE; AND A SECONDTHERMOCOUPLE LOCATED ON THE INSIDE CLOSED END OF SAID ELECTROLYTE TUBESO AS TO DETECT THE TEMPERATURE OF SAID SECOND ELECTRODE.
 2. An in-situoxygen detector as set forth in claim 1 wherein said first and secondelectrode strips overlap in the central portion of the closed end ofsaid tube so as to form an electrode pair for ion conduction on thecenter of the closed end of said tube.
 3. An in-situ oxygen detector asset forth in claim 2 wherein said self-contained reference atmosphereincludes a metal-metal oxide powder, said metal reacting with any oxygenmigrating through said zirconium-oxide tube to form the metal oxide andthereby maintaining said reference atmosphere''s oxygen partial pressureconstant.
 4. An in-situ oxygen detector as set forth in claim 3 whereinsaid sealing means includes the brazing of said zirconia cap to saidelectrolyte tube with a braze alloy capable of maintaining said brazedseal at 1500*F ambient temperature.
 5. An in-situ oxygen detector as setforth in claim 1 wherein said first and second thermocouples arethickfilm thermocouples from 0.1 to 1.0 mils thick
 6. An in-situ oxygendetector as set forth in claim 1 including: means for enclosing saidelectrolyte tube so as to leave the closed end of said tube exposed toan atmosphere of unknown oxygen content; a first means for heating aportion of the enclosed length of said electrolyte tube near the closedend of said tube, said first heating means being located along thelength of said tube and internal to said enclosing means; a second meansfor heating the unknown atmosphere and the closed end of said tubelocated opposite the closed end of said tube; and means for controllingsaid first and second heating means so that the temperature sensed bysaid first and second thermocouples is maintained at a substantIallyindentical temperature level.